... I got her , her mammoth tusk...

I think you need a necklace from the Mara temple in Riften. I'm not sure, but I think I have one and it gives +35 speech. It's not from the temple, though.

go to the temple of mara in riften and talk to the redguard there about marriage.
i belive he will give you a amulet of mara then equip it and talk to ysolda

She's un-marryable unless on PC, which case you have to use CC to be able to :\

This is incorrect. I'm on PS3, with her as my loving yet somewhat annoying wife.

Not true. I was playing on PS3 tonight and gave Ysolda the mammoth tusk (already had the amulet). She was interested in me straight away.

I find that Lydia is the one that cannot be married. Even though she has already moved into Breezehome with me, and I didn't even ask her...:confused:

Then I guess they patched since I last tried :tongue:

As posted above, this is false. I married her and am playing on 360. I didn't quite remember giving her the mammoth tusk, but I know I finished that quest thing. I remember because I kept forgetting and dropping it, causing the the quest topic thing to pop up.

That is the Amulet of Dibella; it gives 15+ to Speech and I think it stacks with the Blessing at her shrines.